1. Field of the Invention
The invention described herein relates to a method for qualitative and quantitative determination of trace impurities in carrier gases, and more specifically to a method wherein the gas to be examined is liquefied by cooling and pressurization and brought to a temperature at which the dissolved impurities have sufficiently narrow absorption spectra to be readily distinguished from one another by ir or uv spectrometry. It is a result of a contract with the Department of Energy.
There are several methods for identifying and determining trace impurities in gases, such as mass spectroscopy, gas chromatography, and infrared spectrometry, or a combination of these methods. Infrared spectrometry has the advantage of being a versatile technique, but it suffers from poor sensitivity and occasionally from poor specificity, i.e., from difficulty in identifying overlapping bands of different compounds when complex mixtures are to be analyzed. Very long pathlengths may be employed to overcome the poor sensitivity, but with certain mixtures, quantitative and often qualitative analysis is still impossible.
In this patent application we present a method for greatly improving the sensitivity and specificity of infrared spectrometry as applied to quantitative determination of some trace impurities in gases by utilizing cryogenic solutions. Solubilities of trace gases in liquefied carrier gases are often sufficiently high to make analyses practical. Since molecular densities in liquids are .about.2.times.10.sup.22 molecules/cm.sup.3, a 1-ppm level of some impurity translates into .gtoreq.10.sup.16 molecules/cm.sup.3 of impurity in solution, much above the molecular density in the gas phase. Molecular rotation is suppressed in solution, so that the generally complex rotational structure of a solute vibrational band usually collapses to a single, sharp feature having a peak absorbance (at moderate resolutions) higher than that for the gas at room temperature. Provided that the liquefied carrier gas is essentially free of infrared absorptions at wavelengths where the impurities absorb, detection of low levels (ppm-ppb) of impurities can be performed.
2. Description of the Prior Art
A careful review of the novelty search material has uncovered one patent. U.S. Pat. No. 2,917,629 (Method for the Analysis of Liquid Chlorine) teaches the investigation of impurities found in chlorine gas by infrared analysis of a liquefied sample. The method described, although superficially similar to the instant invention, has not been sufficiently investigated by the inventor to uncover its real potential. The collapse of the rotation-vibration structure is not mentioned. This spectral simplification is crucial for analysis of complicated mixtures. Further, no mention is made of the facts that the increased densities allow high sensitivity to be trivially achieved, and that the solubilities of the various impurities in the liquefied host gas determine the ultimate utility of the method. The fact that these characteristics are not claimed in this patent is either indicative that the inventor was not aware of or not interested in the possibility of generalizing the method to systems other than chlorine, and in particular that such techniques can be valuable for analyzing mixtures with complex composite features. Finally, the use of an internal mirror and other infrared light directing optics are unnecessary since the cells used for the liquefication and observation in the instant invention fit trivially into standard sample compartments in many commercially available spectrometers.
3. Statement of the Objects
An object of the invention is the demonstration that conventional spectrometry can be used to analyze trace impurities in some gases with considerably greater sensitivity and specificity than is commonly expected if the mixture is liquefied. Other objects and novel features of the invention will become apparent to those skilled in the art upon examination of the following detailed description of a preferred embodiment of the invention and the accompanying drawings.